This invention relates to photomasks and photomask blanks indispensable for production of semiconductors, integrated circuits (IC), large-scale integrated circuits (LSI), color stripe filters, and other electronic products.
As the photomasks, which are original plates for printing very minute circuit patterns on semiconductor wafers and the like, there are inexpensive emulsion masks for users with relatively lower precision and hard masks having excellent durability for uses with high precision such as chromium masks, etc. In the latter hard masks, other than chromium films, chromium oxide films, iron oxide films and silicon films are practically used. Further, for use in the formation of submicron patterns on very-LSI, magnetic bubble devices, surface elastic wave devices and others, there have been developed hard masks with the use of a tantalum film having good dry etching adaptability and excellent durability, which is also a material of low toxicity, as is disclosed by Japanese Laid-open Patent Application No. 65673/1977.
This invention is intended to improve the characteristics of the tantalum multi-layer film hard mask as briefly described above thereby to realize a photomask with higher precision.
In general, hard masks are primarily of the type having a multi-layer film provided as the masking layer such as a two-layer type in which a reflection preventive film is provided on its outer surface for the purpose of avoiding repeated reflection between the mask and the wafer and also increasing the exposure latitude or a three-layer type in which the reflection on the opposite side is also prevented, thus being capable of actuating auto mask aligners. Most of the photomasks fabricated with the use of a tantalum material are also of such a constitution.
However, in tantalum masks having multi-layer structures, similarly as in case of chromium type multi-layer masks, oxide layers are employed as reflection preventive layers. Therefore, while these masks have excellent basic characteristics as described above, a difficulty as described below is encountered in the production thereof and gives rise to problems. That is, a tantalum oxide layer for prevention of reflection, which is ordinarily formed according to the anode oxidation method, is very firm as compared with the pure tantalum masking layer. On the other hand, the etching speed in a tantalum oxide layer is very slow with a Freon type gas such as carbon tetrafluoride, and it is difficult to use a large etching speed ratio relative to the photoresist formed as the upper layer during dry etching.
For this reason, while a wide process latitude is afforded in the case of a single tantalum film without substantial dimensional shift even when applying an overetching longer by two to three times than the just etching time, a dimensional variation similar to that in a chromium type mask will generally occur in case of a multi-layer film laminated with a tantalum oxide film. This is in contradiction to the purpose of high precision uses such as in the formation of submicron patterns. Further, since the etching speed is 4 to 5 times different from those in other layers, it is difficult to apply etching evenly, and irregularities are generally liable to be formed. This will lead under some conditions to defects caused by film residues formed by local underetching. The same applies to the case when the oxide film is formed by the sputtering method with the use of a tantalum oxide target.
As a second problem, there is the difficulty encountered in the formation of films. While the anode oxidation method makes possible uniform and bulk treatment for formation of an oxide film for a tantalum oxide layer, it is a wet process with the use of a chemical reagent and therefore must follow steps different from those of the dry process, such as sputtering employed for formation of a tantalum layer. Since this is a wet process, it entails complications in liquid handling with respect to impurities, foreign matters, particles, and the like, whereby defects are liable to be caused on the surface of a mask blank.
When the tantalum oxide layer is to be formed according to the reactive sputtering method with the use of a tantalum target and a gas mixture of oxygenargon, a substrate temperature ordinarily in the range of from 700.degree. to 900.degree. C. is essentially required. With such a requirement, it is impossible to form a very flat photomask blank with the use of glass as substrate. On the other hand, when the film formation is carried out at a substrate temperature not higher than 300.degree. C., reproducibility of the degree of oxidation is poor, and the speed of the film formation is also very slow. In the production of photomask blanks wherein generation of even a very small defect is a great problem, as judged from the situation as described above, the all dry processes, especially the reactive sputtering method is preferable, but it is difficult to form laminated films of tantalum oxide and tantalum.
In order to eliminate the difference between the etching speeds of the tantalum oxide layer and the tantalum layer, it may be considered possible to vary the film quality of the tantalum oxide film by changing the conditions in the anode oxidation method or the sputtering method such as the liquid composition, gaseous composition, temperature, and current. However, the range of such a variation is very narrow, and a drastic change in conditions may result in impairment of the basic properties such as durability and optical characteristics of the film, or in marked lowering of productivity.